Brake control device

ABSTRACT

A brake control device according to an embodiment executes, for an electric braking wheel of either the front wheel and the rear wheel, a brake hold control for driving an electric braking device to move a propulsion shaft toward the braked member to be braked and bring it into contact with a piston, calculating a target braking force for maintaining the stationary state of the vehicle, subtracting a first braking force from the target braking force to calculate a required hydraulic braking force applied to the non-electric braking wheel different from the electric braking wheel, and controlling a differential pressure control valve connected to the non-electric braking wheel of the hydraulic braking device so that the required hydraulic braking force is applied to the non-electric braking wheel.

TECHNICAL FIELD

The present disclosure relates to a brake control device.

BACKGROUND ART

In recent years, in vehicles such as passenger cars, a brake holdfunction that automatically holds the brake force when the vehicle isstopped by the operation of the brake pedal by the driver has beenwidely adopted. This brake hold function is particularly convenient whenthe vehicle is stopped on a slope.

There have been roughly two methods to realize the brake hold function.The first method is a method of energizing the normally opendifferential pressure control valve in the hydraulic circuit to have itin a closed state when the brake pedal is operated by the driver andhydraulic pressure is applied to the wheels, thus maintaining thehydraulic pressure even after the brake pedal is no longer operated. Thesecond method is a method of calculating and generating the electricbrake force required to maintain the stationary state by ElectricParking Brake (EPB) when the brake pedal is operated by the driver andhydraulic pressure is applied to the wheels.

CITATIONS LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application PublicationNo. 2006-306351

SUMMARY OF INVENTION Technical Problems

In the first method, the differential pressure control valve must becontinuously energized while the vehicle is maintained in a stationarystate. As a result, power consumption is increased. Furthermore, in thesecond method, the electric brake force to maintain the stationary stateis generated independently by the EPB separately from the hydraulicpressure, an extra braking force is generated, and therefore, an extrapower consumption is generated.

Therefore, one of the problems of the present disclosure is to provide abrake control device capable of realizing a brake hold function with lowpower consumption by using the EPB.

Solutions to Problems

A brake control device according to the present disclosure relates to abrake control device applied to a vehicle, the brake control devicecomprising, a hydraulic braking device that makes a braking member pressagainst a braked member rotating integrally with wheels by usinghydraulic pressure so that hydraulic braking force applied to the frontand rear wheels of the vehicle is generated, and an electric brakingdevice that makes the braking member press against the braked member bydriving a motor so that an electric braking force applied to an electricbraking wheel that is either the front wheel and the rear wheel isgenerated, a control unit that, when execution of a brake hold controlfor maintaining a stationary state is permitted in a situation where thevehicle is in the stationary state by the hydraulic braking force,executes the brake hold control in which a propulsion shaft moves towardthe braked member so that the propulsion shaft contacts with a piston bydriving the electric braking device, in which a differential pressurecontrol valve is controlled so that a required hydraulic braking forceapplies to a non-electric braking wheel, the hydraulic braking devicehaving the differential pressure control valve connected to thenon-electric braking wheel different from the electric braking wheel,the required hydraulic braking force calculated by subtracting a firstbraking force from a target braking force to maintain the stationarystate, the first braking force that is the electric braking forceapplied to the electric braking wheel in the absence of hydraulicpressure after the propulsion shaft contacts with the piston.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing an overall outline of a vehiclebraking device according to a first embodiment.

FIG. 2 is a schematic cross-sectional view of a wheel brake mechanism ofa rear wheel system provided in the vehicle braking device of the firstembodiment.

FIG. 3 is a configuration diagram showing a schematic configuration of ahydraulic braking device and an electric braking device according to thefirst embodiment.

FIGS. 4A-4G are a timing chart showing the state of operation of eachconfiguration when the brake hold function is executed in the vehiclebraking device of the first embodiment.

FIG. 5 is a flowchart showing a process executed by the brake controldevice of the first embodiment.

DESCRIPTION OF EMBODIMENT

Hereinafter, exemplary embodiments of the present disclosure, which isfirst embodiment and second embodiment, will be disclosed. Theconfigurations of the embodiment shown below, and the operations andresults (effects) provided by the configurations are merely examples.The present disclosure can also be realized with configurations otherthan the configurations disclosed in the following embodiments.Furthermore, according to the present disclosure, it is possible toobtain at least one of the various effects (including derivativeeffects) obtained by the following configuration.

First Embodiment

In a first embodiment, a vehicle braking device in which a disc brakeEPB is applied to the rear wheel system will be described in thisexample. FIG. 1 is a schematic view showing an overall outline of avehicle braking device of the first embodiment. FIG. 2 is a schematiccross-sectional view of a wheel brake mechanism of the rear wheel systemprovided in the vehicle braking device of the first embodiment.

As shown in FIG. 1 , the vehicle braking device of the first embodimentincludes a service brake 1 that generates a service brake force, whichis hydraulic braking force, in response to the pedaling force of thedriver, and an EPB 2 for regulating the movement of the vehicle when thevehicle is parked or the like.

The service brake 1 is a hydraulic brake mechanism (also referred to asa hydraulic braking device) that generates brake hydraulic pressurebased on the driver's depression of the brake pedal 3 and generatesservice brake force based on this brake hydraulic pressure.Specifically, the service brake 1 boosts the pedaling forcecorresponding to the depression of the brake pedal 3 by the driver witha booster 4, and then generates a brake hydraulic pressure correspondingto the boosted pedaling force in a master cylinder (hereinafter referredto as M/C) 5. Then, the brake hydraulic pressure is transmitted to awheel cylinder (hereinafter, referred to as W/C) 6 provided in a wheelbrake mechanism of each wheel to generate a service brake force.Furthermore, an actuator 7 for controlling brake hydraulic pressure isprovided between the M/C 5 and the W/C 6. The actuator 7 adjusts theservice brake force generated by the service brake 1 and performsvarious controls (e.g., anti-skid control etc.) for improving the safetyof the vehicle.

Various controls using the actuator 7 are executed by an electronicstability control—an electric control unit (ESC-ECU) 8 that controls theservice brake force. For example, the ESC-ECU 8 outputs a controlcurrent for controlling various control valves provided in the actuator7 and a motor for driving the pump to control the hydraulic circuitprovided in the actuator 7, and control the W/C pressure transmitted tothe W/C 6. Wheel slip is thereby avoided, for example, and the safety ofthe vehicle is improved. For example, the actuator 7 is configured toinclude, for each wheel, a pressure increasing control valve thatcontrols the application of the brake hydraulic pressure generated inthe M/C 5 or the brake hydraulic pressure generated by the pump drivewith respect to the W/C 6, a pressure decreasing control valve thatdecreases the W/C pressure by supplying brake fluid in each W/C 6 to areservoir, and the like, and performs pressure increasing, maintaining,and decreasing control of the W/C pressure. Furthermore, the actuator 7can realize the automatic pressurizing function of the service brake 1,and based on the control of the pump drive and various control valves,can automatically pressurize the W/C 6 even when there is no brakeoperation. Details of the hydraulic braking device including theactuator 7 will be described later with reference to FIG. 3 .

On the other hand, the EPB 2 generates electric parking brake force(hereinafter, also referred to as “electric braking force” and “electricbrake force”) by driving the wheel brake mechanism by the EPB motor 10,and is configured to have an EPB-ECU 9 that controls the drive of theEPB motor 10. The EPB-ECU 9 and the ESC-ECU 8 transmit and receiveinformation by, for example, Controller Area Network (CAN)communication.

The wheel brake mechanism is a mechanical structure that generates abrake force in the vehicle braking device of the first embodiment, andhas a structure in which a wheel brake mechanism of the front wheelsystem first generates a service brake force by the operation of theservice brake 1. On the other hand, the wheel brake mechanism of therear wheel system has a common structure that generates a brake forcefor both the operation of the service brake 1 and the operation of theEPB 2. The wheel brake mechanism of the front wheel system is a wheelbrake mechanism that has been conventionally used in general, in which amechanism for generating the electric brake force based on the operationof the EPB 2 is omitted, as opposed to the wheel brake mechanism of therear wheel system, and thus the description thereof will be omittedhere, and the wheel brake mechanism of the rear wheel system will bedescribed below.

In the wheel brake mechanism of the rear wheel system, not only when theservice brake 1 is operated but also when the EPB 2 is operated, thebrake pad 11, which is the friction material shown in FIG. 2 , ispressed to sandwich the brake disc 12 (12RL, 12RR, 12FR, 12FL) which isa friction object material by the brake pad 11, thus generating afriction force between the brake pad 11 and the brake disc 12, andgenerating a brake force.

Specifically, the wheel brake mechanism rotates the EPB motor 10directly fixed to the body 14 of the W/C 6 for pressing the brake pad 11as shown in FIG. 2 in the caliper 13 shown in FIG. 1 to rotate the spurgear 15 provided on a drive shaft 10 a of the EPB motor 10. Then, thebrake pad 11 is moved by transmitting the rotational force (output) ofthe EPB motor 10 to a spur gear 16 engaged with the spur gear 15, andthe electric brake force by the EPB 2 is generated.

In the caliper 13, in addition to the W/C 6 and the brake pad 11, a partof the end face of the brake disc 12 is housed so as to be sandwiched bythe brake pad 11. The W/C 6 can generate the W/C pressure in a hollowportion 14 a, which is the brake fluid storage chamber, by introducingthe brake hydraulic pressure into the hollow portion 14 a of thecylindrical body 14 through a passage 14 b, and is configured to includea rotary shaft 17, a propulsion shaft 18, a piston 19, and the like inthe hollow portion 14 a.

The rotary shaft 17 has one end connected to the spur gear 16 through aninsertion hole 14 c formed in the body 14, so that when the spur gear 16is rotated, the rotary shaft 17 is rotated with the rotation of the spurgear 16. A male screw groove 17 a is formed on the outer peripheralsurface of the rotary shaft 17 at the end of the rotary shaft 17opposite to the end connected to the spur gear 16. On the other hand,the other end of the rotary shaft 17 is axially supported by beinginserted into the insertion hole 14 c. Specifically, the insertion hole14 c is provided with a bearing 21 together with an O-ring 20, so thatthe O-ring 20 prevents the brake fluid from leaking out between therotary shaft 17 and the inner wall surface of the insertion hole 14 c,and the bearing 21 axially supports the other end of the rotary shaft17.

The propulsion shaft 18 is configured by a nut including a hollowtubular member, and has a female screw groove 18 a to be screw fittedwith the male screw groove 17 a of the rotary shaft 17 formed on theinner wall surface. The propulsion shaft 18 is configured, for example,in a circular column shape or a polygonal column shape provided with akey for preventing rotation, so that even if the rotary shaft 17 isrotated, it cannot be rotated about the rotation center of the rotaryshaft 17. Therefore, when the rotary shaft 17 is rotated, the rotationalforce of the rotary shaft 17 is converted to a force for moving thepropulsion shaft 18 in the axial direction of the rotary shaft 17 by theengagement between the male screw groove 17 a and the female screwgroove 18 a. When the drive of the EPB motor 10 is stopped, thepropulsion shaft 18 stops at the same position due to the frictionalforce from the engagement between the male screw groove 17 a and thefemale screw groove 18 a, where if the drive of the EPB motor 10 isstopped when the target electric brake force is obtained, the propulsionshaft 18 can be held at that position, desired electric brake force canbe maintained and self-locking (hereinafter simply referred to as“lock”) can be performed.

The piston 19 is arranged so as to surround the outer periphery of thepropulsion shaft 18, and is formed by a bottomed cylindrical member or apolygonal cylindrical member and arranged such that the outer peripheralsurface comes into contact with the inner wall surface of the hollowportion 14 a formed in the body 14. A structure is such that a sealmember 22 is provided on the inner wall surface of the body 14 and W/Cpressure can be applied to the end face of the piston 19 so that brakefluid does not leak out between the outer peripheral surface of thepiston 19 and the inner wall surface of the body 14. The seal member 22is used to generate a reaction force for returning the piston 19 at thetime of release control after the lock control. Since the seal member 22is provided, basically, even if the brake pad 11 and the piston 19 arepushed in within a range not exceeding the elastic deformation amount ofthe seal member 22 by the tilted brake disc 12 during turning, they arepushed back toward the brake disc 12 so that the gap between the brakedisc 12 and the brake pad 11 is held at a predetermined clearance.

In addition, to prevent the piston 19 from rotating about the rotationcenter of the rotary shaft 17 even if the rotary shaft 17 rotates, whenthe propulsion shaft 18 is provided with a rotation prevention key, thepiston is provided with a key groove in which the key slides, and whenthe propulsion shaft 18 has a polygonal column shape, the piston has apolygonal cylindrical shape corresponding thereto.

The brake pad 11 is arranged at the distal end of the piston 19, and thebrake pad 11 is moved in the left-right direction in the plane ofdrawing accompanying the movement of the piston 19. Specifically, thepiston 19 is movable in the left direction in the plane of drawingaccompanying the movement of the propulsion shaft 18, and is movable inthe left direction in the plane of drawing independently from thepropulsion shaft 18 when the W/C pressure is applied to the end of thepiston 19 (the end opposite to the end where the brake pad 11 isarranged). Then, if the brake hydraulic pressure in the hollow portion14 a is not applied (W/C pressure=0) when the propulsion shaft 18 is atthe release position (the state before the EPB motor 10 is rotated),which is the standby position in the normal release, the piston 19 ismoved in the right direction in the plane of drawing by the elasticforce of the seal member 22 to be described later, and the brake pad 11can be separated away from the brake disc 12. Furthermore, when the EPBmotor 10 is rotated and the propulsion shaft 18 is moved in the leftdirection in the plane of drawing from the initial position, even if theW/C pressure becomes 0, the movement of the piston 19 in the rightdirection in the plane of drawing is regulated by the moved propulsionshaft 18 and the brake pad 11 is held in place.

In the wheel brake mechanism configured as described above, when theservice brake 1 is operated, the piston 19 is moved in the leftdirection in the plane of drawing based on the W/C pressure generatedthereby so that the brake pad 11 is pressed against the brake disc 12and the service brake force is generated. Furthermore, when the EPB 2 isoperated, the spur gear 15 is rotated by driving the EPB motor 10, andthe spur gear 16 and the rotary shaft 17 are accordingly rotated, sothat the propulsion shaft 18 is moved toward the brake disc 12 (leftdirection in the plane of drawing) based on the engagement between themale screw groove 17 a and the female screw groove 18 a. The distal endof the propulsion shaft 18 thereby comes into contact with the bottomsurface of the piston 19 and presses the piston 19, whereby the piston19 is also moved in the same direction, so that the brake pad 11 ispressed against the brake disc 12 and an electric brake force isgenerated. Therefore, a shared wheel brake mechanism that generates abrake force for both the operation of the service brake 1 and theoperation of the EPB 2 can be adopted.

Furthermore, it is possible to confirm the generation state of theelectric braking force by the EPB2 or recognize the current detectionvalue by confirming the current detection value of the current sensor(not shown) for detecting the current through the EPB motor 10.

A longitudinal acceleration sensor 25 detects acceleration in thelongitudinal direction (traveling direction) of the vehicle and inputs adetection signal to the EPB-ECU 9.

An M/C pressure sensor 26 detects the M/C pressure in the M/C 5 andinputs a detection signal to the EPB-ECU 9.

A temperature sensor 28 detects the temperature of the wheel brakemechanism (e.g., a brake disc) and inputs a detection signal to theEPB-ECU 9.

A wheel speed sensor 29 detects the rotation speed of each wheel andinputs a detection signal to the EPB-ECU 9. Although the wheel speedsensor 29 is actually provided one for each wheel, detailed illustrationand description thereof will be omitted here.

The EPB-ECU 9 is configured by a well-known microcomputer equipped withCentral Processing Unit (CPU), Read Only Memory (ROM), Random AccessMemory (RAM), I/O, and the like, and performs parking brake control bycontrolling the rotation of the EPB motor 10 following the programstored in the ROM and the like.

The EPB-ECU 9 inputs, for example, a signal corresponding to theoperation state of an operation switch (SW) 23 provided on aninstrumental panel (not shown) in the vehicle compartment, and drivesthe EPB motor 10 according to the operation state of the operation SW23. Furthermore, the EPB-ECU 9 executes lock control, release control,and the like based on the current detection value of the EPB motor 10,and recognizes that the lock control is being performed based on thecontrol state or that the wheel is in the lock state by the lockcontrol, and that the release control is being performed or that thewheel is in the release state. or EPB release state, by the releasecontrol. Then, the EPB-ECU 9 outputs a signal for performing variousdisplays to an indicator lamp 24 provided on the instrumental panel.

The vehicle braking device configured as described above basicallyperforms an operation of generating a braking force in the vehicle bygenerating the service brake force by the service brake 1 when thevehicle is traveling. Moreover, when the vehicle is stopped by theservice brake 1, the driver performs operations such as pressing theoperation SW 23 to operate the EPB 2 and generate the electric brakeforce thus maintaining the stationary state, and then releasing theelectric brake force thereafter. That is, as the operation of theservice brake 1, when the driver operates the brake pedal 3 while thevehicle is traveling, the brake hydraulic pressure generated in the M/C5 is transmitted to the W/C 6 thus generating the service brake force.Moreover, as the operation of the EPB 2, the piston 19 is moved bydriving the EPB motor 10, and the electric brake force is generated bypressing the brake pad 11 against the brake disc 12 to have the wheelsin the lock state, or the electric brake force is released by separatingthe brake pad 11 from the brake disc 12 to have the wheels in therelease state.

Specifically, the electric brake force is generated or released by thelock/release control. In the lock control, the EPB 2 is operated byforward rotating the EPB motor 10, the rotation of the EPB motor 10 isstopped at a position where a desired electric brake force can begenerated by the EPB 2, and this state is maintained. A desired electricbrake force is thereby generated. In the release control, the EPB 2 isoperated by reverse rotating the EPB motor 10, and the electric brakeforce generated in the EPB 2 is released.

FIG. 3 is a configuration diagram showing a schematic configuration ofthe hydraulic braking device and the electric braking device accordingto the first embodiment. As illustrated in FIG. 3 , the vehicle brakingdevice according to the first embodiment includes a hydraulic brakingdevice 60 configured to be able to apply a braking force (frictionbraking torque) to four wheels 50RL, 50RR, 50FR, and 50FL, and an EPB 2(FIG. 1 ) including the EPB motor 10 configured to be able to apply abraking force to two wheels 50RL and 50RR.

The hydraulic braking device 60 includes four wheel cylinders 6,pressure adjusting units 34RL, 34RR, 34FR and 34FL, and a refluxmechanism 37. Each of the four wheel cylinders 6 is a mechanism thatpressurizes the brake pads (FIG. 1 ) to apply braking force to thewheels 50RL, 50RR, 50FR, and 50FL. The pressure adjusting units 34RL,34RR, 34FR and 34FL are mechanisms for adjusting the hydraulic pressureapplied to the corresponding wheel cylinder 6, respectively. The refluxmechanism 37 is a mechanism that returns the fluid (working fluid)serving as a medium for generating the hydraulic pressure toward theupstream side. The differential pressure control valves 33R and 33F openand close under the control of the ESC-ECU 8 (see FIG. 1 ).

The pressure adjusting units 34RL, 34RR, 34FR, and 34FL each includeselectromagnetic valves 35 and 36 capable of electrically switchingbetween the open state and the closed state. The electromagnetic valves35 and 36 are provided between the differential pressure control valve33 and a reservoir 41. The electromagnetic valve 35 is connected to thedifferential pressure control valves 33R, 33F, and the electromagneticvalve 36 is connected to the reservoir 41.

The electromagnetic valves 35 and 36 open and close under the control ofthe ESC-ECU 8 to increase, maintain, or decrease the pressure generatedby the wheel cylinder 6.

The reflux mechanism 37 includes the reservoir 41 and a pump 39, and apump motor 40 that rotates the front-side and rear-side pumps 39 totransport the fluid toward the upstream side. One of each of thereservoir 41 and the pump 39 is provided in correspondence with thecombination of the pressure adjusting units 34RL and 34RR and thecombination of the pressure adjusting units 34FR and 34FL.

Here, in the first embodiment, the EPB motor 10 driven under the controlof the EPB-ECU 9 (FIG. 2 ) is connected to each of the two wheelcylinders 6 on the rear side. Thus, in the first embodiment, the brakepads 11 (FIG. 2 ) of the two wheel cylinders 6 on the rear side arepressurized in response to the drive of the EPB motor 10, so that theelectric braking force is applied to the wheels 50RL and 50RR on therear side. Therefore, in the first embodiment, the two wheel cylinders 6on the rear side and the two EPB motors 10 connected to these two wheelcylinders 6 function as EPB 2 capable of generating a parking brakingforce separate from the hydraulic braking force by the hydraulic brakingdevice 60.

Here, the details of the control of the ESC-ECU 8 and the EPB-ECU 9 willbe described. The ESC-ECU 8 and the EPB-ECU 9 are applied to a vehicleincluding a hydraulic braking device that presses the brake pad 11(braking member) with hydraulic pressure toward the brake disc 12(member to be braked) that rotates integrally with the wheels andgenerates a hydraulic braking force, for the front and rear wheels ofthe vehicle, and an electric braking device that presses the brake pad11 by driving the EPB motor 10 toward the brake disc 12 and generates anelectric braking force, for an electric braking wheel of either thefront wheel and the rear wheel.

Then, when the execution of the brake hold control for maintaining astationary state is permitted in a situation where the vehicle ismaintained in the stationary state by the hydraulic braking forcegenerated by the hydraulic braking device 60, the ESC-ECU 8 and theEPB-ECU 9 execute the following brake hold control.

The EPB-ECU 9 drives the EPB2 to move the propulsion shaft 18 toward thebrake disc 12 and bring it into contact with the piston 19 of the rearwheels, which is electric braking wheels, and calculates a targetbraking force for maintaining the stationary state of the vehicle. Inaddition, the EPB-ECU 9 calculates a required hydraulic braking forceapplied to the front wheels, which is non-electric braking wheels, bysubtracting a value of a first braking force that is the electricbraking force is generated by the EPB2 in the absence the hydraulicbraking force after the propulsion shaft 18 contacts with the piston 19from the target braking force. The ESC-ECU 8 controls the differentialpressure control valve 33F (FIG. 3 ) in the hydraulic braking device 60connected to the front wheels to generate the required hydraulic brakingforce.

Furthermore, when a value of the required hydraulic braking forceobtained by subtracting the first braking force in the absence thehydraulic braking force after the propulsion shaft 18 contacts with thepiston 19 from the target braking force is less than or equal to zero,the ESC-ECU 8 controls the differential pressure control valve 33F inthe hydraulic braking device 60 connected to the front wheels so that nohydraulic braking force is generated on the front wheels.

Moreover, when the hydraulic braking force generated by the hydraulicbrake operation increases during the execution of the brake holdcontrol, the ESC-ECU 8 and the EPB-ECU 9 again execute the brake holdcontrol from the beginning.

Next, with reference to FIG. 4 , the state of operation of eachconfiguration when the brake hold function is executed in the firstembodiment will be described. FIG. 4 is a timing chart showing the stateof operation of each configuration when the brake hold function isexecuted by the vehicle braking device of the first embodiment. In FIGS.4A to 4G, the horizontal axis represents time. In FIG. 4(a), thevertical axis represents the hydraulic brake operation amount (theamount of depression of the brake pedal 3). In FIG. 4(b), the verticalaxis represents the presence/absence (ON/OFF) of the brake hold (BH)instruction (BH start operation by the operation SW 23). In FIG. 4C, thevertical axis represents the energized state (energized/de-energized) ofthe differential pressure control valve 33F connected to the frontwheels. In FIG. 4D, the vertical axis represents the energized state(energized/de-energized) of the differential pressure control valve 33Rconnected to the rear wheels. In FIG. 4E, the vertical axis representsthe current value (current detection value) through the EPB motor 10. InFIG. 4F, the vertical axis represents the braking force applied to thefront wheels. In FIG. 4G, the vertical axis represents the braking forceapplied to the rear wheels.

It is assumed that while the vehicle is in the stationary state, asshown in FIG. 4A, the driver performs the first hydraulic brakeoperation from time t1 to time t7, and then performs the secondhydraulic brake operation stronger than the first time from time t8 totime t14. Furthermore, as shown in FIG. 4B, it is assumed that thedriver gives a BH instruction (BH start operation by the operation SW23) at time t3, and then the ON state of the BH instruction continues.

In that case, the EPB-ECU 9 drives the EPB2 to operate the EPB motor 10to move the propulsion shaft 18 (FIG. 2 ) toward the brake disc 12 andbring it into contact with the piston 19 provided with the rear wheels(time t3 to t4 in FIG. 4E). Thus, for the rear wheels, even if thehydraulic pressure applied to the rear wheels decreases thereafter(after time t5 in FIG. 4 A), an electric braking force substantially thesame as the hydraulic braking force up to that point can be generated(FIG. 4G).

Next, the EPB-ECU 9 calculates the target braking force for maintainingthe stationary state of the vehicle. For example, the target brakingforce is determined by using the road gradient calculated from thedetection values of the longitudinal acceleration sensors. In addition,the EPB-ECU 9 calculates the required hydraulic braking force applied tothe front wheels by subtracting the first braking force that is theelectric braking force is generated by EPB2 in the absence the hydraulicbraking force after the propulsion shaft 18 contacts with the piston 19from the target braking force.

Then, the ESC-ECU 8 controls the differential pressure control valve 33F(FIG. 3 ) in the hydraulic braking device connected to the front wheels60 so that the required hydraulic braking force is applied to the frontwheels. Thus, even if the hydraulic brake operation amount starts todecrease from time t5 and becomes zero at t7 (FIG. 4A), the brakingforce applied to the front wheels starts to decrease from time t5 andbecomes the required hydraulic braking force at time t6, and thereafter,maintains the required hydraulic braking force (FIG. 4F).

Moreover, when the driver performs the second hydraulic brake operationstronger than the first time from time t8 to time t14 during theexecution of the brake hold control, the ESC-ECU 8 and the EPB-ECU 9again execute the brake hold control from the beginning. That is, theEPB-ECU 9 first drives the EPB2 to operate the EPB motor 10 to move thepropulsion shaft 18 (FIG. 2 ) toward the brake disc 12 and bring it intocontact with the piston 19 for the rear wheels (time t11 to t12 in FIG.4E). Thus, for the rear wheels, even if the hydraulic pressure appliedto the rear wheels decreases thereafter (after time t13 in FIG. 4 A), anelectric braking force substantially the same as the hydraulic brakingforce up to that point can be generated (FIG. 4G). That is, the electricbraking force after time t13 can be made larger than the electricbraking force from time t5 to time t10 (FIG. 4G).

In addition, the EPB-ECU 9 calculates the required hydraulic brakingforce applied to the front wheels by subtracting the first braking forcegenerated by EPB2 in the absence of hydraulic pressure after thepropulsion shaft 18 contacts with the piston 19 from the target brakingforce. At this time, when the calculated required hydraulic brakingforce is less than or equal to zero, the ESC-ECU 8 controls thedifferential pressure control valve 33F in the hydraulic braking device60 connected to the front wheels so that no hydraulic braking force isapplied to the front wheels. As a result, as shown in FIG. 4F, thebraking force applied to the front wheels starts to decrease from timet13 and becomes zero at time t14. However, after time t14, even if thebraking force applied to the front wheels is zero (FIG. 4F), the brakingforce applied to the rear wheels is large (FIG. 4G), and the brakingforce for maintaining the stationary state is secured for the vehicle asa whole.

Next, the process executed by the brake control device will be describedwith reference to FIG. 5 . FIG. 5 is a flowchart showing a processexecuted by the brake control device of the first embodiment.

First, the driver starts the first hydraulic brake operation (step S1 inFIG. 5 : time t1 in FIG. 4A).

Next, when the operation unit (operation SW 23) is operated by thedriver to instruct the execution of the brake hold function (step S2 inFIG. 5 : time t3 in FIG. 4B), the EPB-ECU 9 drives the EPB2 to operatethe EPB motor 10 to move the propulsion shaft 18 toward the brake disc12 and bring it into contact with the piston 19 connected to the rearwheels (step S3 in FIG. 5 : time t3 to t4 in FIG. 4E).

Next, the EPB-ECU 9 calculates the target braking force for maintainingthe stationary state of the vehicle (step S4 in FIG. 5 ). Next, theEPB-ECU 9 calculates the first braking force generated by the EPB2 inthe absence of hydraulic pressure (step S5 in FIG. 5 ). For example, thefirst braking force is the same as the hydraulic braking force after thepropulsion shaft 18 contacts with the piston 19.

Next, the EPB-ECU 9 calculates the required hydraulic braking force bysubtracting the electric braking force calculated in step S5 from thetarget braking force calculated in step S4 (step S6 in FIG. 5 ). Next,the ESC-ECU 8 controls the differential pressure control valve 33F (FIG.3 ) in the hydraulic braking device 60 connected the front wheels sothat the required hydraulic braking force calculated in step S6 isapplied to the front wheels (step S7 in FIG. 5 ).

Next, the driver terminates the first hydraulic brake operation (step S8in FIG. 5 : time t7 in FIG. 4A). According to the above control, asshown in FIG. 4G, the braking force applied to the rear wheels ismaintained even after time t5 when the driver starts to loosen thehydraulic brake operation. Furthermore, as shown in FIG. 4F, the brakingforce applied to the front wheels is maintained by the requiredhydraulic braking force from time t6 to time t9.

The driver then initiates a second hydraulic brake operation (step S9 inFIG. 5 : time t9 in FIG. 4A).

Next, when the hydraulic brake operation amount reaches its peak (timet11 in FIG. 4A), the EPB-ECU 9 drives the EPB2 to operate EPB motor 10to move the propulsion shaft 18 toward the brake disc 12 and bring itinto contact with the piston 19 for the rear wheels (step S10 in FIG. 5: time t11 to t12 in FIG. 4E).

Next, the EPB-ECU 9 calculates the first braking force that is theelectric braking force, which can be identical with the originalhydraulic braking force, generated by the EPB2 in the absence ofhydraulic pressure (step S11 in FIG. 5 ).

Next, the EPB-ECU 9 calculates the required hydraulic braking forceapplied to the front wheels by subtracting the first braking forcecalculated in step S11 from the target braking force calculated in stepS4 (step S12 in FIG. 5 ). Next, the ESC-ECU 8 controls the differentialpressure control valve 33F (FIG. 3 ) in the hydraulic braking device 60connected to the front wheels so that the required hydraulic brakingforce calculated in step S12 is applied to the front wheels (step S13 inFIG. 5 ).

Next, the driver terminates the second hydraulic brake operation (stepS14 in FIG. 5 : time t14 in FIG. 4A). According to the above control, asshown in FIG. 4G, the braking force applied to the rear wheels ismaintained even after time t13 when the driver starts to loosen thehydraulic brake operation. Furthermore, as shown in FIG. 4F, the brakingforce applied to the front wheels becomes zero after time t14.

As described above, according to the brake control device of the firstembodiment, the brake hold function can be realized with low powerconsumption by using the EPB2. That is, for example, as shown in FIG.4C, the differential pressure control valve 33F connected to the frontwheels can be energized from time t3 to time t12. On the other hand, inthe first method of the prior art described above, the differentialpressure control valve connected to the front wheels had to be energizedwhile the BH instruction is ON (FIG. 4B), resulting in large powerconsumption.

Furthermore, as shown in FIG. 4D, it is not necessary to energize thedifferential pressure control valve 33R connected to the rear wheels. Onthe other hand, in the first method, the differential pressure controlvalve connected to the rear wheels had to be energized while the BHinstruction was ON (FIG. 4B), resulting in large power consumption.

L2 shown in FIG. 4G is a target value of the braking force applied tothe rear wheels in the first method. According to the brake controldevice of the first embodiment, the braking force applied to the rearwheels larger than L2 can be maintained even after the time t5 when thedriver starts to loosen the hydraulic brake operation by simply drivingthe EPB motor 10 from time t3 to time t4, and thus it is efficient.Furthermore, larger braking force applied to the rear wheels can bemaintained even after the time t13 when the driver starts to loosen thehydraulic brake operation by simply driving the EPB motor 10 from timet11 to time t12, and thus it is efficient.

L1 shown in FIG. 4F is a target value of the braking force applied tothe front wheels in the first method. According to the brake controldevice of the first embodiment, the braking force applied to the rearwheels can be maintained larger than L2 from time t6 to time t9 (FIG.4G), and hence the braking force applied to the front wheels (FIG. 4F)can be made smaller than L1, whereby power consumption can be reducedaccordingly.

Furthermore, in the second method of the prior art described above,there is a problem that since the electric brake force required tomaintain the stationary state is calculated and generated independentlyby the EPB separately from the hydraulic pressure, an extra brakingforce is generated, and therefore, an extra power consumption isgenerated. However, according to the brake control device of the firstembodiment, when the brake hold function is executed, the hydraulicbraking force applied to the front wheels is reduced by the amount ofthe large electric braking force applied to the rear wheels, so thatsuch extra braking force and power consumption are not generated.

Furthermore, in the prior art, there is a method of holding thehydraulic braking force for a predetermined time and then switching tothe electric braking force when executing the brake hold function, butthe differential pressure control valve needs to be continuouslyenergized during a predetermined time for holding the hydraulic brakingforce, resulting in large power consumption. On the other hand,according to the brake control device of the first embodiment, whenexecuting the brake hold function, the differential pressure controlvalve connected to the rear wheels does not need to be energized at all,and the differential pressure control valve connected to the frontwheels also does not need to be energized after time t12 in the exampleof FIG. 4 , and thus the power consumption can be prevented small.

Moreover, in the first method, even if the differential pressure controlvalve is continuously energized in order to maintain the stationarystate, due to structural reasons, the fluid gradually may pass throughthe differential pressure control valve, and as the hydraulic pressuregradually decreases, the braking force decreases, and it may not bepossible to maintain the stationary state for a long time. On the otherhand, according to the brake control device of the first embodiment,since such a decrease does not occur with the electric braking force bythe EPB2, the stationary state can be maintained even for a long time.

Second Embodiment

Next, a brake control device of the second embodiment will be described.The description on the matters same as in the first embodiment will beomitted as appropriate.

In the brake control device of the second embodiment, when the servicebrake 1 fails during the execution of the brake hold control, theEPB-ECU 9 controls the EPB2 so that the electric braking force becomesthe target braking force for the rear wheels.

Thus, even when the service brake 1 fails, stable braking control can berealized by controlling the EPB2 so that the electric braking forcebecomes the target braking force.

The embodiment and modified examples of the present disclosure have beendescribed above, but the above-described embodiments and modifiedexamples are merely examples, and they are not intended to limit thescope of the disclosure. The novel embodiments and modified examplesdescribed above can be implemented in various forms, and variousomissions, substitutions, or modifications can be made without departingfrom the gist of the disclosure. Furthermore, the embodiments andmodified examples described above are included in the scope and gist ofthe disclosure, and are included in the disclosure described in theClaims and the equivalent scope thereof.

For example, in the embodiments described above, the rear wheels areelectric braking wheels, but this is not the sole case, and the frontwheels may be electric braking wheels.

Moreover, the hydraulic circuit is a so-called front-rear piping asshown in FIG. 3 (piping configuration in which the output from the M/C 5is divided into two systems, two front wheels and two rear wheels), butthis is not the sole case, and the hydraulic circuit may be a so-calledX piping (piping configuration in which the output from the M/C 5 isdivided into two systems of front and rear wheels on a diagonal line).When the X piping is adopted, both of the two differential pressurecontrol valves need to be energized when executing the brake holdfunction, but similar to the example of FIG. 4 where the differentialpressure control valve of the front wheels is de-energized after timet12, the two differential pressure control valves can also bede-energized from the middle of the execution of the brake holdfunction, and hence the power consumption can be suppressed to be small.

The present disclosure can also be applied at the time of execution ofthe brake hold function in an autonomous vehicle.

Furthermore, in the embodiments described above, a mode in which thepresence/absence of the instruction of the brake hold control isswitched by the driver operating the operation unit is shown. Instead,the brake hold control may be executed when the control unit determinesthat the brake hold control is necessary regardless of the driver'sintention or operation.

Furthermore, in the embodiments described above, a mode in which theelectric braking device is operated to execute the brake hold control ina situation where the hydraulic braking force is generated by operatingthe brake pedal is shown. Instead, the brake hold control may beexecuted in a situation where the hydraulic braking force isautomatically generated by the control unit when the brake pedal is notoperated.

The invention claimed is:
 1. A brake control device applied to avehicle, the brake control device comprising: a hydraulic braking devicethat makes a braking member press against a braked member rotatingintegrally with wheels by using hydraulic pressure so that hydraulicbraking force applied to the front and rear wheels of the vehicle isgenerated; an electric braking device that makes the braking memberpress against the braked member by driving a motor so that an electricbraking force applied to an electric braking wheel that is either thefront wheel and the rear wheel is generated; and a control unit that,when execution of a brake hold control for maintaining a stationarystate is permitted in a situation where the vehicle is in the stationarystate by the hydraulic braking force, executes the brake hold control inwhich a propulsion shaft moves toward the braked member so that thepropulsion shaft contacts with a piston by driving the electric brakingdevice, in which a differential pressure control valve is controlled sothat a required hydraulic braking force applies to a non-electricbraking wheel, the hydraulic braking device having the differentialpressure control valve connected to the non-electric braking wheeldifferent from the electric braking wheel, the required hydraulicbraking force calculated by subtracting a first braking force from atarget braking force to maintain the stationary state, the first brakingforce is the electric braking force applied to the electric brakingwheel in the absence of hydraulic pressure after the propulsion shaftcontacts with the piston.
 2. The brake control device according to claim1, wherein if the required braking force is less than or equal to zero,the control unit controls the differential pressure control valve sothat no hydraulic braking force is applied to the non-electric brakingwheel.
 3. The brake control device according to claim 1, wherein whenthe hydraulic braking force increases during the execution of the brakehold control, the control unit executes the brake hold control onceagain.
 4. The brake control device according to claim 1, wherein whenthe hydraulic braking device fails during the execution of the brakehold control, the control unit controls the electric braking device sothat the target braking force is applied to the electric braking wheel.5. The brake control device according to claim 2, wherein when thehydraulic braking force increases during the execution of the brake holdcontrol, the control unit executes the brake hold control once again. 6.The brake control device according to claim 2, wherein when thehydraulic braking device fails during the execution of the brake holdcontrol, the control unit controls the electric braking device so thatthe target braking force is applied to the electric braking wheel. 7.The brake control device according to claim 3, wherein when thehydraulic braking device fails during the execution of the brake holdcontrol, the control unit controls the electric braking device so thatthe target braking force is applied to the electric braking wheel. 8.The brake control device according to claim 1, wherein the first brakingforce is identical with the hydraulic braking force when the propulsionshaft contacts with the piston in the stationary state.